1. Field of the Invention
The present invention relates to a method and device for touch position detection, and more particularly, to a method and device for touch position detection on large-size surface acoustic wave panel.
2. Description of the Prior Art
Surface acoustic wave (SAW) touch panel is a touch panel that determines the position of a touch input on a touch screen by sensing a SAW signal at a target location. It converts an electrical signal to the SAW signal by using a transducer including a piezoelectric material, and then determines whether the SAW signal is blocked and cannot be received when traveling on the touch screen.
FIG. 1A is a schematic diagram illustrating the structure of a conventional SAW touch panel. As shown in FIG. 1A, a touch panel 10 includes a screen area 11 and a reflection area 12. The reflection area 12 includes a sensing device 13, which has a first and a second horizontal-axis transducer element 14a and 14b and a first and a second vertical-axis transducer element 15a and 15b, wherein the second horizontal-axis and vertical axis transducer elements 14b and 15b receive SAW signals Signal_V1 and Signal_V2 corresponding to input electrical signals Signal_Ei1 and Signal_Ei2 sent by the first horizontal-axis and vertical axis transducer elements 14a and 15a, respectively. In addition, the sensing device 13 further includes a set of first and second vertical-axis reflection units 16a and 16b and a set of first and second horizontal-axis reflection units 17a and 17b. These four reflection units 16a, 16b, 17a and 17b each includes a plurality of reflectors r. These reflectors r are all partially transmissive and partially reflective. Meanwhile, the SAW signals Signal_V1 and Signal_V2 necessary for sensing a possible touch point P input on each horizontal axis and vertical axis are provided by this partially transmissive and partially reflective effect of the reflectors r. These reflectors r can be a wiring layer printed on a glass substrate of the touch screen, so cost of manufacturing is low. In addition, the reflectors r of the reflection units 16a, 16b, 17a and 17b are all arranged from sparse to dense (as seen from the traveling directions of the SAW signals Signal_V1 and Signal_V2). The reason for this is because that, in the case of evenly arranged reflection units 16a, 16b, 17a and 17b, the SAW signals Signal_V1 and Signal_V2 available for reflection for reflectors r at the back are less due to partial reflection. This affects the ability of the reflection units 16a, 16b, 17a and 17b to accurately sense the positions of input touch points corresponding to the back parts thereof. Thus, the reflection units 16a, 16b, 17a and 17b are arranged from sparse to dense to even the SAW signal Signal_V1 or Signal_V2 input to each reflector r for compensation. FIGS. 1B and 1C are diagrams illustrating electric potentials of output electrical signals Signal_Eo1 and Signal Eo2 of the SAW touch panel shown in FIG. 1A without and with a touch point P input, respectively. In the diagrams, Vy represents the electrical potential of the output electrical signal Signal_Eo1, and is the X axis of the coordinate of the input touch point P; Vx represents the electrical potential of the output electrical signal Signal_Eo2, and is the Y axis of the coordinate of the input touch point P. The reason that the duration of Vx is longer than that of Vy is because the path traveled by the second SAW signal Signal_V2 is longer. The depression shown in FIG. 1C is a representation of the sensing of the touch point P, which is the basis for determining the position of the touch point P input. In addition, at the beginning of Vy and Vx, there may be a spike (not shown) caused by the input electrical signals Signal_Ei1 and Signal_Ei2 being received by the second horizontal-axis transducer element 14b and the second vertical-axis transducer element 15b directly via the second horizontal-axis reflection unit 17b and the second vertical-axis reflection unit 16b immediately after input, respectively.
However, in large-size SAW touch panels, since the surface acoustic wave attenuates with the increase of the propagation distance and the number of reflections traversed, as shown in FIG. 1E, the size of the depression also reduces. Thus, it is possible that a touch further away from the horizontal-axis and vertical-axis transducer elements 14a and 15b is not detected because the size of the depression fails to exceed a threshold.
From the above it is clear that prior art still has shortcomings. In order to solve these problems, efforts have long been made in vain, while ordinary products and methods offering no appropriate structures and methods. Thus, there is a need in the industry for a novel technique that solves these problems.